"My (I hope well educated) opinion: We can get to a sustainable world of 10 billion smiling people.... if we really want to."
"We can double our energy, and we can do it in a sustainable way."
We rely heavily on energy use, dominated by finite fossil fuels. We have high scientific confidence, based on solid physics, that burning most of the remaining fossil-fuel resource and releasing the carbon dioxide will cause large and long-lasting climate changes. Studies of societal and economic impacts typically indicate that such large climate changes will make life notably more difficult for future generations, and a measured response starting soon is economically favorable. Uncertainties are substantial, but with larger or faster climate changes more likely than smaller or slower ones. Fortunately, sustainable energy resources are abundant, and extensive use can be achieved with existing technologies or logical extensions thereof, allowing the economically optimal shift away from fossil fuels.